Zeus is mainly here due to the Re-Move+1 leader skill. Using it your units is allowed to retreat or charge in enemy mobs at your disposal, plus I see his passive skill like the Fullface (7 star), 1 time gale, partnered with it's max stats.

Brynhildr 7 star is actually just an average unit. Range is 1-2, flying unit what more can we need? So movement is great, but mainly due to her Victory skill, which add Opponents Level minus Units Level to the attack value. Overall her stats aren't that good though.

Heroine is good unit with 1-2 range. Her skill ups her own Crit chance by 20 if correct and is stackable in battle.

Elephanter (yes the archer riding the elephant), here is added due to her Training skill which makes all units move faster in the turn order.

Basically the tierlist is agreeable to an extent but unit skillset and stats are seemingly what matter more. I'll probably make a blog out of a unit tier list of own soon (hopefully).